It's the same package for every Windows and includes DX9, DX10 in 32bit and 64bit flavors. Redistributable version that is.

edit: last August version had Aug2008_d3dx9_39_x64.cab, Aug2008_d3dx9_39_x86.cab, Aug2008_d3dx10_39_x64.cab, Aug2008_d3dx10_39_x86.cab, Aug2008_XACT_x64.cab, Aug2008_XACT_x86.cab, Aug2008_XAudio_x64.cab, Aug2008_XAudio_x86.cab and same sort of stuff all the way back to 2005, hence it's size.